Passing QFile to function - qt

I get an illegal indirection error at generateCSVHeader(*file4);.
function declaration:
void generateCSVHeader(QFile * file);
function use:
str="MyData.csv";
QFile file4(str);
generateCSVHeader(*file4);
when I drop the dereference designator, it gives me a cannot convert QFIle to QFile * error.

You should pass pointer to your QFile object (which is an address) instead of passing the object itself (dereferencing is irrelevant, because it is used only with pointers, not objects). To get an address of your object, you should use the & operator. So you have to invoke your function like this:
generateCSVHeader(&file4)
Also, you may consider using references instead of pointers.

Related

Returning object reference from C++ function

I have began learning C++ for Arduino and have run into some troubles.
I have some functions reading/writing to SPIFFS files.
Since the code for opening configuration files is common I would want to have a separate function to handle it.
I have come up with following function declaration
ConfigFileOpenStatus DeviceOpenConfigFile(const char *path, File *file);
The function accepts pointer to char array for the file path, and pointer to opened file.
I then tried to make following function definition
ConfigFileOpenStatus DeviceOpenConfigFile(const char *path, File *file)
{
if (SPIFFS.exists(path))
{
file = &SPIFFS.open(path, "r+");
return !file ? Failed : Opened;
}
else
{
file = &SPIFFS.open(path, "w+");
return !file ? Failed : Created;
}
}
That did not work as compiler complained with error error: taking address of temporary [-fpermissive]
As I understand this means that the file object will be disposed once DeviceOpenConfigFile function returns?
So my question is whether its possible to implement a function in a way where I can get File object reference and release it later?
SPIFFS.open apparently returns File, by value. The returned value will be a temporary variable available on that line. So taking the address of that one doesn't make any sense, for the same reason as int func (void); ... &func() doesn't make any sense. It has nothing to do with the surrounding DeviceOpenConfigFile function.
It doesn't make sense to assign a new address to pointer passed by parameter, for the same reason as void func (int x) { x = 0; } doesn't make sense - you change a local variable only, nothing on the caller side gets changed and nothing gets returned to the caller.
It would seem that the solution you are looking for is this:
ConfigFileOpenStatus DeviceOpenConfigFile(const char *path, File* file)
{
...
*file = SPIFFS.open(path, "r+");
where file is allocated on the caller-side.

How to call function from COM correctly in QT?I

I want to call a function from COM like this:
long func(int count,Variant *buffer);
Programmers should allocate float array and deliver the pointer and length to function func.
What I do is like this:
QAxobject myCom;
float[1024] buffer;
QVariant v=QVariantfromValue(buffer);
myCom.setControl("{........}");
myCom.dynamicCall("func(int,Variant*)",1024,v);
When exe runs, I find it not works as I assumed.Then how to call func correctly?

overloading + operator in QString ,Qt

I want to overload the + operator for my mainwindow class(or any other class) for Qstring.
this is what I have done so far :
void operator+(QString a,QString b)
{
qDebug()<<"works";
}
but the thing is, the QString + operator is already overloaded(to concatenate, I guess).
so, if I use the above code, it results in ambiguity(both the signatures are same).
how do I override the actual function to my own function without making a new class to hold QString?
Try QString operator+(QString a,QString b). The return type of the overloaded function is QString, not void. The compiler cannot differ between two overloads only differ by return type.

why do strings have pointers as there return type?

I know what pointers are but when it comes to strings/arrays I get really confused. If someone has an answer or a website that explains it that would be great. For example:
char * strncopy (char*dest, char * source, size_t);
Why the pointer? what is it pointing to? Does it a pointer usually store an address?
It is sayed in my textbook that each string building function is of type pointer char*.
Also I was trying to see if I could write a program that would clear things up, but it didn't work. Can someone tell me how to fix it, or what I'm doing wrong.
#include <stdio.h>
#include <string.h>
char * getname ()
{
char name [10];
scanf ("%s", name);
return (name);
}
int main (void)
{
char name[10];
printf ("Enter your name\n");
name[] = getname();
printf ("Hi %s", name);
return (0);
}
Inside of your getname function, when you return a pointer to the name array because it's allocated on the stack it gets destroyed leaving you with an invalid pointer. Dereferencing such a pointer causes many, many problems.
You should allocate the name array inside of getname on the heap, with malloc/calloc so that when you return the pointer the data won't be destroyed.
With regards to functions like strncpy, they tend to return a pointer to the resulting string; e.g.: strncpy returns a pointer to the destination.
Pointer itself represents an address, e.g. if you have a pointer typed char *pstr, you can always check the underlying address with printf("address of my pointer %p\n", pstr);
In C programming language, a string is an array of char. If you have a good knowledge of array and its memory layout, it's not too hard for you to understand c-styled string. Generally speaking, an array in C is a continuous chunk of memory with name of array represent address of the first element in the array. So is string who is a chunk of memory with name of the char array address of the first character. In addition, c-styled string terminates with character \0, so if you want to manage memory for string yourself, remember one extra byte for the tailing \0.
As to your second problem, your name in function getname is a local variable whose life time ends when function returns. However, you still want to access name outside the function which is inappropriate. You can solve this be dynamically allocated memory like in dasblinkenlight's and others' post.
Good luck.

CLI/C++: void* to System::Object

This is a similar question to this SO post, which I have been unable to use to solve my problem. I have included some code here, which will hopefully help someone to bring home the message that the other posting was getting at.
I want to write a CLI/C++ method that can take a void pointer as a parameter and return the managed object (whose type I know) that it points to. I have a managed struct:
public ref struct ManagedStruct { double a; double b;};
The method I am trying to write, which takes a void pointer to the managed struct as a parameter and returns the struct.
ManagedStruct^ VoidPointerToObject(void* data)
{
Object^ result = Marshal::PtrToStructure(IntPtr(data), Object::typeid);
return (ManagedStruct^)result;
}
The method is called here:
int main(array<System::String ^> ^args)
{
// The instance of the managed type is created:
ManagedStruct^ myData = gcnew ManagedStruct();
myData->a = 1; myData->b = 2;
// Suppose there was a void pointer that pointed to this managed struct
void* voidPtr = &myData;
//A method to return the original struct from the void pointer
Object^ result = VoidPointerToObject(voidPtr);
return 0;
}
It crashes in the VoidPointerToObject method on calling PtrToStructure , with the error: The specified structure must be blittable or have layout information
I know this is an odd thing to do, but it is a situation I have encountered a few times, especially when unmanaged code makes a callback to managed code and passes a void* as a parameter.
(original explanation below)
If you need to pass a managed handle as a void* through native code, you should use
void* voidPtr = GCHandle::ToIntPtr(GCHandle::Alloc(o)).ToPointer();
// ...
GCHandle h = GCHandle::FromIntPtr(IntPtr(voidPtr));
Object^ result = h.Target;
h.Free();
(or use the C++/CLI helper class gcroot)
Marshal::PtrToStructure works on value types.
In C++/CLI, that means value class or value struct. You are using ref struct, which is a reference type despite use of the keyword struct.
A related problem:
void* voidPtr = &myData;
doesn't point to the object, it points to the handle.
In order to create a native pointer to data on the managed heap, you need to use pinning. For this reason, conversion between void* and Object^ isn't as useful as first glance suggests.

Resources